Reflectionless surface wave transducer

ABSTRACT

An improved interdigital surface wave transducer which eliminates unwanted signals due to reflections to a degree sufficient to permit its use as a television band-pass filter. The reflectionless surface wave transducer is obtained by using a modified form of unidirectional transducer in which an interdigital transducer is placed in the center of a U-shaped multi-strip coupler with the transducer phased off center by a distance Delta d so as to satisfy an equation which will result in zero reflections.

States artmann et al.

atent [191 [4 1 Jan. 7, 1975 REFLECTIONLESS SURFACE WAVE TRANSDUCER [75] Inventors: Clinton S. Hartmann, Dallas, Tex.;

Michael J. Birch, Northampton, England [73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Dec. 28, 1973 [21] Appl. No.: 429,258

[52] US. Cl 333/72, 3lO/9.8, 333/30 R [51] Int. Cl H03h 9/02, H03h 9/26, H03h 9/32 [58] Field of Search 333/30 R, 72; 3lO/8.0,

[56] References Cited OTHER PUBLICATIONS Marshall, et al., Theory and Design of the Surface Acoustic Wave Multistrip Coupler in IEEE Transactions on Sonics and Ultrasonics, Vol. su-20 No.2 April 1973; PP. 124433.

Primary Examiner-James W. Lawrence Assistant ExaminerMarvin Nussbaum Attorney, Agent, or Firm-Harold Levine; James T. Comfort; William E. l-liller [57] ABSTRACT An improved interdigital surface wave transducer which eliminates unwanted signals due to reflections to a degree sufficient to permit its use as a television band-pass filter. The reflectionless surface wave transducer is obtained by using a modified form of unidirectional transducer in which an interdigital transducer is placed in the center of a U-shaped multi-strip coupler with the transducer phased off center by a distance A 01 so as to satisfy an equation which will result in zero reflections.

5 Claims, 2 Drawing Figures LOS LOAD

(OB USEC EA.) AND 315 MHZ INSERTION' T.TS.

'2 UNIDlRECTlONAL TRANSDUCERS WITH'REFLECTJON CANCELLATION AT 40 MHZ rJ m PATENTEU A 7 I975 IN MHZ FREQUENCY REFLECTIONLESS SURFACE WAVE TRANSDUCER BACKGROUND OF THE INVENTION This invention relates to acoustic surface wave devices in general and more particularly to an improved interdigital surface wave transducer.

Surface wave acoustic devices are gaining widespread use as filters, delay lines and the like. In particular, in frequency ranges between 10 mhz and 1 ghz, devices which are compact and provide numerous advantages over inductive capacitive type filters and tuned electromagnetic wave guides are possible. This results directly from the fact that acoustic waves travel at a much slower speed than electromagnetic waves and thus, the size of a structure can be correspondingly smaller in the order of 10 When used in filtering applications these devices generally comprise a piezoelectric substrate on which are deposited two spaced transducers. The most common type of transducer used is what is known as the interdigital transducer wherein a plurality of fingers extend from a transducer pad on each side of the substrate and have overlapping portions. Electric fields created between the overlapping fingers of the transducer excite the piezoelectric material to generate the surface waves. In order to obtain the proper filter response, weighting of the interdigital fingers is necessary. The manner of designing such filters is described in a paper published in the IEEE Transactions on Microwave Theories and Techniques entitled Impulse Model Design of Acoustic Surface Wave Filters by C. S. Hartmann, D. T. Bell, .Ir., and R. C. Rosenfeld,.Vol. MTT-2l No. 4, April 1973, pp. 162-175. In the design method described therein, the impulse response is used with the desired frequency response converted into a time response through the use of Fourier transforms and the weighting then done in accordance with the time response obtained.

Typical interdigital surface wave transducers always reflect at least a small portion of any acoustic signal which they receive. Thesize of the reflective signal depends on the electrical load connected to the transducer. Under typical loading, the reflected signal level is approximately the square of the acoustic to electric conversion efficiency. Thus, for example, if the transducer conversion efficiency is -6 dB, then the reflected acoustic signal is 12 dB. When it is desired to use transducers of this nature in a filter to be used as a television band-pass filter, reflected signals at these levels become extremely detrimental. The reflected signals bounce back and forth between the input and output transducers and cause multiple outputs which appear as ghosts on the T.V. picture. In order to avoid such ghosts, suppression of at least 46 dB for T.V. applications is necessary. Without any further measures and using conventional transducers, this amount of suppression, Le, -46 dB requires that the filter be designed so that it has at least a 23 dB insertion loss. Although such operation is possible, this large insertion loss must then be compensated for by additional amplification stages.

Thus, it can be seen that there is a need for a transducer in which the reflective signal level will be sufficiently suppressed without simultaneously increasing the insertion loss.

SUMMARY OF THE INVENTION This problem is solved in the present invention through the use of a modified form of a unidirectional transducer. Such a transducer is disclosed in the article Surface Acoustic Wave Multi-Strip Components and their Applications by F. G. Marshall, C. 0. Newton and E. S. G. Paige, published in the IEEE Transactions on Sonics and Ultra-Sonics SU20 No. 2, pp. 134 to 143. The device disclosed in that article is one in which complete conversion from acoustic to electrical energy is desired. Thus, it is for the case of a matched electrical load. Such matching is not possible as a practical matter in the case of a television IF filter since such BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of the transducer arrangement of the present invention.

FIG. 2 is a response curve for a transducer according to the present invention illustrating suppression of reflected signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates the arrangement of the present invention. Shown is the output interdigital transducer 11 having connected across it a load 13. Transducer 11 comprises transducer pads 15 from which extend interleaved digital fingers 17. As shown, the interdigital transducer 11 is placed in the center of a multi-strip U- shaped coupler l9. Couplers of this nature aid in obtaining unidirectional operation. That is to say, without the coupler, a certain amount of the energy incident on the interdigital transducer would not be converted, but would continue on past the transducer. The use-of the coupler 19 causes a certain portion of the energy to be picked up by the coupler on its side 21 and transmitted to its side 23 where it will further excite the substrate so that the energy received by ther interdigital transducer fingers 17 will be the sum of the energy which passed through the side 21 to the fingers 17 and that which was coupled from the side 21 to the side 23.

As noted above, a certain amount of the energy will also be reflected. What the arrangement of the present invention does is to use the multi-strip coupler 19 in such away that the total reflected energy will have a phase relationship such that components from the interdigital transducer 17 and the multi-strip coupler 19 will tend to cancel each other out thereby substantially reducing the level of reflected signals. It has been found that reflections will be substantially zero when the following equation is satisfied.

ae W

Where p is the ratio of the acoustic transmission coefficient to the acoustic reflection coefficient, both of which depend on the electrical load impedance and are generally complex numbers, a is the coupling efficiency of the U-shaped multi-strip coupler 19 and A d is the off-set of the interdigital transducer 11 from the center of the U-shaped coupler 19, i.e., as illustrated in FIG. 1, A d equals zi -d The manner of computing the types of coeffcients described above is well-known in the art. For example, see Acoustic Surface Wave Filters by R. H. Tancrell and M. G. Holland published in the Proceedings of the IEEE, Vol. 59, No. 3, March, 1971. For general background see Acoustic Fields and Waves in Solids by B. A. Auld, John Weilly and Sons, 1973.

FIG. 2 illustrates the triple transit suppression in a device constructed using two transducers such as that shown on FIG. 1, i.e., an input and output transducer are mounted on opposite ends of a substrate. Triple transit refers to the number of trips a wave makes. Consider a signal coming from a transducer to the left of the transducer 11 of FIG. 1. It will make one transit to get to the transducer. The reflected portion, however will travel back to the input transducer and then back again to the output transducer. Thus, for this signal, which is the interfering signal, a triple transit over the distance between the two transducers will have occurred. Thus, the reference to triple transit suppression. The curve of FIG. 2 is for one transducer such as that on FIG. 1 designed for reflection cancellation at 40 mhz and the other for reflection cancellation at 35 mhz. The solid-line indicates the insertion loss for the directly transmitted signal. As shown, this insertion loss between the frequency of 35 and 40 mhz is approximately 1 2 dB. At these frequencies the corresponding triple transit suppression is approximately down to -46 dB, the amount of suppression required in an application such as this.

Thus, an improved transducer arrangement which provides reflectionless operation materially improving the suppression of reflected signals has been shown. While the reflectionless surface wave transducer in accordance with this invention has been described with specific emphasis upon its application in a television IF filter, it will be understood that it has general applicability in filters, such as delay line filters, frequency dispersive filters, bandpass filters, and phase coded filters, for example.

Although a specific embodiment has been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the spirit of the invention which is intended to be limited solely by the appended claims.

What is claimed is:

1. A surface wave device comprising: a piezoelectric substrate; a multi-strip coupler disposed on said sub strate, said coupler including a pair of spaced apart substantially parallel arms connected together at one end thereof by an integral bight portion, an interdigital transducer disposed in the space between said arms of said coupler with a preselected off-set from the center of said space, the position of said interdigital transducer being selected to satisfy the equation:

where p is the ratio of the acoustic transmission coefficient to the acoustic reflection coefficient, a is the coupling efficienty of the multi-strip coupler, and Ad is the off-set of the interdigital transducer from the center of the space between said arms of the coupler.

2. A surface wave device according to claim I, wherein said multi-strip coupler is U-shaped.

3. A surface wave device according to claim I and further including a second transducer on said substrate in spaced relation to the first transducer, one of said transducers being an input transducer and the other transducer being an output transducer.

4. A surface wave device according to claim 3 wherein one transducer is substantially reflectionless at a first frequency and the other transducer is substantially reflectionl'ess at a second frequency, said surface wave device having a filter response characterized by a main lobe, and both of said first and second frequencies being within the main lobe of the filter response.

5. A surface wave device according to claim 4 wherein said device is a television IF filter. 

1. A surface wave device comprising: a piezoelectric substrate; a multi-strip coupler disposed on said substrate, said coupler including a pair of spaced apart substantially parallel arms connected together at one end thereof by an integral bight portion, an interdigital transducer disposed in the space between said arms of said coupler with a preselected off-set from the center of said space, the position of said interdigital transducer being selected to satisfy the equation: Alpha ej d square root 1 - Rho 2 where Rho is the ratio of the acoustic transmission coefficient to the acoustic reflection coefficient, Alpha is the coupling efficienty of the multi-strip coupler, and Delta d is the off-set of the interdigital transducer from the center of the space between said arms of the coupler.
 2. A surface wave device according to claim 1, wherein said multi-strip coupler is U-shaped.
 3. A surface wave device according to claim 1 and further including a second transducer on said substrate in spaced relation to the first transducer, one of said transducers being an input transducer and the other transducer being an output transducer.
 4. A surface wave device accOrding to claim 3 wherein one transducer is substantially reflectionless at a first frequency and the other transducer is substantially reflectionless at a second frequency, said surface wave device having a filter response characterized by a main lobe, and both of said first and second frequencies being within the main lobe of the filter response.
 5. A surface wave device according to claim 4 wherein said device is a television IF filter. 